The present invention relates to a process for the preparation of 4,5-diamino shikimic acid derivatives, especially for the preparation of ethyl (3R, 4R, 5S)-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate and its pharmaceutically acceptable addition salts from 4-amino-5-azido shikimic acid derivatives, especially from ethyl (3R, 4R, 5S)-4-acetamido-5-azido-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate.
4,5-diamino shikimic acid derivatives, especially the ethyl (3R, 4R, 5S)-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate and its pharmaceutically acceptable addition salts are potent inhibitors of viral neuraminidase ( J. C. Rohloff et al., J.Org.Chem. 63, 1998, 4545-4550; WO 98/07685).
A reduction of ethyl (3R, 4R, 5S)-4-acetamido-5-azido-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate to ethyl (3R, 4R, 5S)-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate by a hydrogenation in the presence of a Raney nickel catalyst is known in the art (J. C. Rohloffet al, loc.cit.).
It was found that the xe2x80x9c5-azidoxe2x80x9d starting compound from its prior synthesis always contains a small amount of the xe2x80x9c2,5-diazidoxe2x80x9d compound formed by formal addition of hydrazoic acid to the double bond. In the course of the hydrogenation the azido group in 5-position is readily converted to the desired amino group, the transformation of the azido group in 2-position however is very slow. Accordingly a xe2x80x9c2-azido-5-aminoxe2x80x9d intermediate is formed which was shown to be xe2x80x9cAmes positivexe2x80x9d and therefore suspicious of being mutagenic.
This intermediate cannot be satisfactorily removed with the common purification techniques. Also, the problem cannot be overcome by prolonging the hydrogenation time because the xe2x80x9ccyclohexene double bondxe2x80x9d becomes hydrogenated, too.
An object of the present invention is, therefore, to provide a process for the preparation of 4,5-diamino shikimic acid derivatives which does not encompass the difficulties known in the art; i.e. a process which allows easy access to the target product in an excellent quality.
It was found that reduction of 4-amino-5-azido shikimic acid derivatives with a phosphine in the presence of a carboxylic acid surprisingly achieved this object.
The present invention therefore relates to a process for the preparation of a 4,5-diamino shikimic acid derivative of formula 
and a pharmaceutically acceptable addition salt thereof
wherein
R1 is an optionally substituted alkyl group,
R2 is an alkyl group and
R3 and R4, independent of each other are H or an amino protecting group, with the proviso that not both R3 and R4 are H;
the process being characterized by the reduction of a 4-amino-5-azido-shikimic acid derivative of formula 
xe2x80x83with a phosphine in the presence of a carboxylic acid. R1, R2, R3 and R4 have the same meaning in formula II as in formula I. If necessary, the process includes a further transformation of the 4,5-diamino shikimic acid derivative into a pharmaceutically acceptable addition salt thereof.
The term alkyl in R1 has the meaning of a straight chained or branched alkyl group of 1 to 20 C-atoms, expediently of 1 to 12 C-atoms. Examples of such alkyl groups are methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, tert. butyl, pentyl and its isomers, hexyl and its isomers, heptyl and its isomers, octyl and its isomers, nonyl and its isomers, decyl and its isomers, undecyl and its isomers and dodecyl and its isomers.
This R1 alkyl group can be substituted with one or more substituents as defined in e.g. WO 98/07685. Suitable substituents are C1-6-alkyl (as defined above), C1-6-alkenyl, C3-6-cycloalkyl, hydroxy, C1-6-alkoxy, C1-6-alkoxycarbonyl, F, Cl, Br, and I. Preferred meaning for R1 is 1-ethylpropyl.
R2 is a straight chained or branched alkyl group of 1 to 12 C-atoms, expediently of 1 to 6 C-atoms as exemplified above.
The preferred meaning for R2 is ethyl.
The term amino protecting group in R3 and R4 refers to any protecting group conventionally used and known in the art. They are described e.g. in xe2x80x9cProtective Groups in Organic Chemistryxe2x80x9d, Theodora W. Greene et al., John Wiley and Sons Inc., New York, 1991, p.315-385. Suitable amino protecting groups are also given in e.g. WO 98/07685.
Preferred amino protecting groups are alkanoyl groups, more preferably lower C1-6-alkanoyl such as hexanoyl, pentanoyl, butanoyl (butyryl), propanoyl (propionyl), ethanoyl (acetyl) and methanoyl (formyl). The preferred alkanoyl group, and therefore preferred meaning for R4, is acetyl. The preferred meaning for R4 is H.
The 4-amino-5-azido-shikimic acid derivative of formula (II) as starting compounds of the present process of the invention are accessible as described in J. C. Rohloff et al., loc. cit. and in WO 98/07685.
The preferred 4-amino-5-azido-shikimic acid derivative of formula (II) is the ethyl (3R, 4R, 5S)-4-acetamido-5-azido-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate. Accordingly, the preferred 4,5-diamino shikimic acid derivative of formula (I) is the ethyl (3R, 4R, 5S)-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate and the corresponding salt, ethyl (3R, 4R, 5S)-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate Phosphate (1:1).
The phosphine used can be defined by the formula
P(R5)3xe2x80x83xe2x80x83III
wherein R5 is alkyl.
R5 expediently is a straight chained or branched C1-8 alkyl group as defined above.
Phosphines which can suitably be used are trioctyl phosphine, triisobutyl phosphine, tri-n-butyl phosphine, and triethyl phosphine. The most preferred phosphine is tri-n-butyl phosphine.
Although the ratio of phospine to the 4-amino-5-azido-shikimic acid derivative of formula (II) is not critical to the production of the desired 4,5-diamino shikimic acid derivative of formula (I), the phosphine is preferably added in stoichiometric amounts or in a slight excess of up to 1.05 equivalents relating to the starting amount of the 4-amino-5-azido-shikimic acid derivative of formula (II). One of skill in the art may adjust to relative amounts of phosphine and the 4-amino-5-azido-shikimic acid derivative of formula (II) to optimize them for the particular reaction conditions used.
Typically, the reduction is performed in a polar protic solvent which forms the reaction medium. Any conventional polar protic solvent can be used, such as alcohols, preferably aqueous ethanol or aqueous tetrahydrofuran, most preferably aqueous ethanol. However, the choice of solvent is not critical to production of the desired 4,5-diamino shikimic acid derivative of formula (I), and one of skill would be able to perform the reduction in other solvents using general knowledge of the art.
The reaction temperature is another non-critical variable; for instance, the reduction performs satisfactorily at room temperature. The preferred reaction temperature mainly depends on the phosphine used but most preferably lies in the range of xe2x88x9220xc2x0 C. to 30xc2x0 C., with between 0 and 25xc2x0 C. being particularly preferred.
It can be favorable to perform the reaction at two temperature levels, thereby having the lower temperature range given above for the addition of the phosphine and thereafter having a slightly higher temperature of up to room temperature to bring the reaction to completion.
Catalytic amounts of a carboxylic acid present during the reduction were found to suppress the ester hydrolysis which otherwise takes place to a small extent of some percent and thereby leads to an undesirable impurity. The term xe2x80x9ccarboxylic acidxe2x80x9d refers to any compound having one or more free carboxylic acid groups. Preferably, the carboxylic acid is an aliphatic carboxylic acid, having from 2 to 8 carbon atoms, such as acetic acid, oxalic acid, propionic acid, malonic acid, butyric acid, succinic acid, maleic acid, fumaric acid, valeric acid, glutaric acid, caproic acid, adipic acid, heptanoic acid, and caprylic acid.
The carboxylic acid is preferably present in the reduction reaction in an amount which measurably reduces the quantity of ester hydrolysis products present after the reaction of a 4-amino-5-azido-shikimic acid derivative of formula (II) with a phosphine to produce the 4,5-diamino shikimic acid derivative of formula (I). More preferably, the carboxylic acid is added to the reaction in quantities from about 0.5 to about 5.0 mol % of the starting amount of the 4-amino-5-azido-shikimic acid derivative of formula (II). Most preferably, the carboxylic acid is added to the reaction in quantities from about 0.5 to about 3.0 mol % of the starting amount of the 4-amino-5-azido-shikimic acid derivative of formula (II). Particularly preferred is the addition of carboxylic acid to the reaction in an amount of about 1.0 mol % of the starting amount of the 4-amino-5-azido-shikimic acid derivative of formula (II).
Preferably, one of skill in the art will be able to adjust the amount of a particular carboxylic acid added to the reaction according to the number of free carboxylic acid groups and the pKa value of the particular acid. Higher pKa acids will be required in relatively larger amounts than lower pKa acids.
Expediently, acetic acid, usually in the form of glacial acetic acid, is added in catalytic quantities of 0.5 mol % to 3.0 mol % of the 4-amino-5-azido-shikimic acid derivative of formula (II).
Although the time allowed for the reaction is not critical, generally, the reduction reaction is complete after 3 to 6 hours.
Thereafter work up of the reaction mixture can happen by applying methods known to those skilled in the art. Expediently the reaction mixture is, preferably after stabilization with xe2x89xa65 mol % acetic acid, concentrated in vacuo.
Though the 4,5-diamino shikimic acid derivative can be isolated e.g. by evaporation and crystallization, it is preferably kept in e.g. an ethanolic solution and then further transformed into the pharmaceutically acceptable addition salt following the methods described in J. C. Rohloff et al., J.Org.Chem. 63, 1998, 4545-4550; WO 98/07685).
The term xe2x80x9cpharmaceutically acceptable acid addition saltsxe2x80x9d embraces all conventionally used salts for pharmaceutical preparations, including salts with inorganic and organic acids, such as hydrochloric acid, hydrobromic acid, nitric acid, sulfuric acid, phosphoric acid, citric acid, formic acid, fumaric acid, maleic acid, acetic acid, succinic acid, tartaric acid, methanesulfonic acid, p-toluenesulfonic acid and the like.
The salt formation is effected with methods which are known per se and which are familiar to any person skilled in the art. Not only salts with inorganic acids, but also salts with organic acids come into consideration. Hydrochlorides, hydrobromides, sulphates, nitrates, citrates, acetates, maleates, succinates, methan-sulphonates, p-toluenesulphonates and the like are examples of such salts.
Preferred pharmaceutically acceptable acid addition salt is the 1:1 salt with phosphoric acid which can be formed preferably in ethanolic solution at a temperature of xe2x88x9220xc2x0 C. to 60xc2x0 C.